1. Field of the Invention
This invention relates to methods and apparatus for surveying for nodular targets present on the sea floor, and more particularly to such method and apparatus adapted to survey for manganese nodules present on the deep sea floor.
2. Description of the Prior Art
In a known method of continuously surveying for nodular targets such as, for example, manganese nodules present on the deep sea floor at a depth of 4,000 to 6,000 meters, the sea floor is continuously photographed with a television camera, or is acoustically investigated by using a sonar suspended near the sea floor. Such a method is described, for example, in JA-OS 69074/53 laid open for public inspection on June 20, 1978. In such a method, a device lowered several thousand meters below the sea level from a ship is towed with a cable while being maintained at a fixed height from the sea floor. However, there are problems in that the surveying speed is restricted to 1 to 2 knots, and that much time is required for survey over a wide range.
There is also known a method of investigating the topography and geological structure of the sea floor wherein sound waves are radiated directly from a ship, and reflected sound waves from the sea floor and the boundary surface of the subsurface layer below the sea floor are observed. The system data are then analyzed to determine the state of the subsurface layer of the sea floor. In the case of granular nodular targets which are distributed and scattered on the sea floor, as no method of analyzing sound waves reflected by such targets has been established, no underwater sonic method of directly surveying for them from a ship has yet been practiced.